This invention relates to a method and apparatus for imaging an internal section (planar slice) through a specimen of tissue, and particularly to tomographic imaging of a section of soft tissue in vitro or in vivo.
A great deal of attention has been given recently to a medical tomographic reconstruction technique for X-rays known as computerized axial tomography (CAT). The first usable system was designed for making tomographs of the head. Although of relatively low resolution (about 3 mm as compared to 0.2 mm for the conventional X-ray shadowgram), that system has produced significant additional information for X-ray diagnosis since it recovers information which would otherwise be lost by integration in the shadowgram. Another well-known diagnostic technique, that of pulse-echo ultrasonics, also produces an image. In this case, however, the image is created by brightening a CRT screen at a position corresponding to a reflecting tissue interface in the body. The result is thus not a tomographic image.
In a recent paper, "Algebraic Reconstruction of Spatial Distributions of Acoustic Absorption within Tissue from Their Two-Dimensional Acoustic Projections," Acoustical Holography, Vol. 5. Plenum Press, New York, 1974, pp. 591-603, Greenleaf, et al. describe an attempt to produce a tomographic image from individual acoustical attenuation measurements using pulsed ultrasonics in the transmission mode and the computerized algebraic reconstruction technique (ART). This is an ultrasonic analog of the CAT method. The ART method was not wholly successful because of refraction and reflection of sound which interfered with the computer reconstruction and prevented an accurate tomograph from being achieved.
Traditionally, the word tomograph has referred to an X-ray picture of a selected plane section of a solid object. The advent of ultrasonic imaging using pulse-echo techniques led to a format that was geometrically identical to the X-ray tomograph in the choice of coordinates, but was an image of ultrasound reflection properties and not those of electromagnetic absorption. By convention, this type of image came to be known as an ultrasonic tomograph.
Up to the present time, this duplication of terminology could cause no confusion because of the different techniques involved (reflection vis-a-vis absorption). However, the recent introduction of image reconstruction methods capable of generating a sectional view from transilluminated projections may become a source of some confusion, particularly in the case of ultrasonic systems. (The prefix "trans-" as used herein indicates illumination through rather than over an object.) This is because there are now three basic types of tomographic images; one using X-rays and two using ultrasonics. A fourth type utilizing radioisotopes as, for example, described by Budinger, et al., "Three-Dimensional Reconstruction in Nuclear Medicine Emission Imaging," IEE Transactions on Nuclear Science, Vol. NS-21, pp. 2-20, 1974, is sufficiently different as to not warrant discussion. The information contained within these images is complementary. With very few exceptions, the images will show different information. One is not a replacement for the other.
The differences among the three tomographs may be seen by inspecting the three images of the same section of the body. One of these will be a conventional X-ray tomograph, and will show the differential attenuation of tissue for X-rays passing through that tissue. The second will be a conventional ultra-sonic pulse-echo tomograph, and will outline the boundaries between tissue of different acoustic impedance by indicating the amount of sound reflected back from those boundaries. The third, to which the present invention pertains, will be an ultrasonic tomograph made by reconstructing the information obtained by the passage of ultrasonic energy completely through the section. The information is the differential attenuation of sound through different types of tissue.
With the exception of the external boundaries of the subject and a few dominant structural characteristics, these three types of tomographs probably will not look alike. This is not to imply that any one is better than another, but rather to imply that the kind of information contained within them is diffferent. Each type of tomograph can reveal a great deal of information to someone experienced in interpreting that particular type of image. What is potentially more significant, however, is the synergism that may occur with two or three types of tomograph, each revealing its own perculiar set of information. This could lead to a diagnostic capability not available from the use of any single type of tomograph.
The class of tomograph to which this invention pertains is that obtained from a measure of the ultrasonic energy that passes completely through the tissue. This is an ultrasonic attenuation (or transmission) tomograph and, in only a general sense, is the acoustic analog of the X-ray tomograph. Because of the more complete control and processing that can take place with ultrasound signals, a great deal more relative information is obtained from this type of ultrasonic tomograph than is obtained from X-ray tomographs alone.
It is anticipated, as an example, that the attenuation ultrasonic tomograph will be particularly useful in detecting tissue lesions. In the case of scirrhous carcinoma in the breast, the tumor mass boundary is somewhat difficult to ascertain by echo ultrasonic tomography but an attenuation tomographic image should be able to show the carcinoma. Differentiation between the carcinoma and the surrounding reactive fibrous tissues should be achievable if there is a difference in absorption between the two regions. It is further anticipated that an attenuation tomograph will show differences between cystic and solid masses, which are not readily available from a reflection tomograph. The fact that ultrasonic transmission images can be made through obliquely oriented tissue boundaries, whereas echo systems must have the ultrasonic beam perpendicular to more boundaries in order for them to be seen, indicates that the continuous demarcation between different tissues will be a distinguishing feature of attenuation tomography.
On the other hand, it has to be recognized that there are some fundamental difficulties in the passage of ultrasound through the body that will always set a limit on the applicability of attenuation tomography. These limitations are those set by the phenomena of refraction, reflection, scattering absorption and dispersion of ultrasound in body tissues. To date, the only practical method of overcoming any of these limitations has been the use of pulse-echo ranging and imaging. The concepts of time delay spectrometry (TDS) described in U.S. Pat. No. 3,466,652, which will be discussed in detail hereinafter, provides the present inventors with a powerful technique enabling them to overcome some of these limitations. The technique has been shown to yield ultrasonic projection shadowgraphs (attenuation images) approaching the theoretical limit (about 1.5 mm) for the system described in a paper by Heyser (supra). The present invention utilizes that existing system, and modifications of that system produce ultrasonic attenuation tomographs.